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Greenwich clock with standard measurements

Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is commonly used in practice to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone, especially by bodies connected with the United Kingdom, such as the BBC World Service,[1] the Royal Navy, the Met Office and others, although strictly UTC is an atomic time scale which only approximates GMT with a tolerance of 0.9 second. It is also used to refer to Universal Time (UT), which is a standard astronomical concept used in many technical fields and is referred to by the phrase Zulu time.

In the UK, GMT is the official time only during winter; during summer British Summer Time is used. GMT is substantially equivalent to Western European Time.[2]

Noon Greenwich Mean Time is not necessarily the moment when the noon sun crosses the Greenwich meridian (and reaches its highest point in the sky in Greenwich) because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (this discrepancy is known as the equation of time). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.

Historically the term GMT has been used with two different conventions for numbering hours. The old astronomical convention (before 1 January 1925) was to refer to noon as zero hours, whereas the civil convention during the same period was to refer to midnight as zero hours. The latter convention is modern practice (on and after 1 January 1925) for astronomical as well as civil purposes. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.

Contents

History

As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT in order to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees (this convention was internationally adopted in the International Meridian Conference of 1884). Note that the synchronization of the chronometer on GMT did not affect shipboard time itself, which was still solar time. But this practice, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, eventually led to GMT being used worldwide as a reference time independent of location. Most time zones were based upon this reference as a number of hours and half-hours "ahead of GMT" or "behind GMT".

Greenwich Mean Time was adopted across the island of Great Britain by the Railway Clearing House in 1847, and by almost all railway companies by the following year, from which the term "railway time" is derived. It was gradually adopted for other purposes, but a legal case in 1858 held "local mean time" to be the official time. This changed in 1880, when GMT was legally adopted throughout the island of Great Britain. GMT was adopted on the Isle of Man in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted Greenwich Mean Time in 1916, supplanting Dublin Mean Time.[3] Hourly time signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the time ball at the observatory obsolete in the process.

The daily rotation of the Earth is somewhat irregular (see ΔT) and is slowing down slightly; atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT was replaced as the international time reference by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world. Universal Time (UT), a term introduced in 1928, initially represented mean time at Greenwich determined in the traditional way to accord with the originally-defined universal day; then from 1 January 1956 (as decided by the IAU at Dublin, 1955, at the initiative of William Markowitz) this 'raw' form of UT was re-labeled UT0 and effectively superseded by refined forms UT1 (UT0 equalized for the effects of polar wandering[4]) and UT2 (UT1 further equalized for annual seasonal variations in earth rotation rate). Leap seconds are nowadays added to or subtracted from UTC to keep it within 0.9 seconds of UT1.

Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by 'the centre of the transit instrument at the Observatory at Greenwich'. Although that instrument still survives in working order, it is no longer in use and now the meridian of origin of the world's longitude and time is not strictly defined in material form but from a statistical solution resulting from observations of all time-determination stations which the BIPM takes into account when co-ordinating the world's time signals. Nevertheless, the line in the old observatory's courtyard today differs no more than a few metres from that imaginary line which is now the Prime Meridian of the world."[5]

Greenwich Mean Time in legislation

Several countries throughout the world legislatively define their local time by explicit reference to Greenwich Mean Time.[6][7] Some examples are:

  • United Kingdom: The Interpretation Act 1978, section 9 - provides that whenever an expression of time occurs in an Act, the time referred to shall (unless otherwise specifically stated) be held to be Greenwich mean time. Under subsection 23(3), the same rule applies to deeds and other instruments.[8][3]
  • Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.[6]
  • Republic of Ireland: Standard Time (Amendment) Act, 1971, section 1,[9] and Interpretation Act 2005, section 18(i).[10]
  • Canada: Interpretation Act, R.S.C. 1985, c. I-21, section 35(1).[11]

Time zone

In the United Kingdom civil time is legally still based on GMT, not UTC, but in common practice UTC is used, the difference of less than a second being negligible for most purposes, and broadcast time signals, though still sometimes known as the Greenwich Time Signal, are based on UTC.[12] The time scale of broadcast time signals in winter is UTC but often still popularly called GMT.

Those countries marked in dark blue on the map above use Western European Summer Time and advance their clock one hour in summer. In the United Kingdom, this is known as British Summer Time (BST); in the Republic of Ireland it is called Irish Standard Time (IST)[13] — officially changing to GMT in winter. Those countries marked in light blue keep their clocks on UTC/GMT/WET year round.

Discrepancies between legal GMT and geographical GMT

Since legal, political, and economic in addition to purely physical or geographical, criteria are used in the drawing of time zones, it follows that actual time zones do not precisely adhere to meridian lines. The 'GMT' time zone, were it drawn by purely geographical terms, would consist of exactly the area between meridians 7°30'W and 7°30'E. As a result, there are European locales that despite lying in an area with a 'physical' UTC time, actually use another time zone (UTC+1 in particular); contrariwise, there are European areas that use UTC, even though their 'physical' time zone is UTC-1 (e.g., most of Portugal), or even UTC−2 (the westernmost part of Iceland). Actually, because the UTC time zone in Europe is 'shifted' to the west, Lowestoft in Suffolk, East Anglia, England at only 1°45'E is the easternmost settlement in Europe in which UTC is applied. Following is a list of the 'incongruencies':

Countries (or parts thereof) west of 22°30'W ('physical' UTC-2) that use UTC
  • The westernmost part of Iceland, incl. the northwest peninsula and its main town of Ísafjörður, which is west of 22°30'W, uses UTC. Bjargtangar, Iceland is the westernmost point in which UTC is applied.
Countries (or parts thereof) west of 7°30'W ('physical' UTC-1) that use UTC
Countries (mostly) between meridians 7°30'W and 7°30'E ('physical' UTC) that use UTC+1
This arch indicates the Greenwich mean time line in Spain
  • Spain (except for the Canary Islands which use UTC). Parts of Galicia in fact lie west of 7°30'W ('physical' UTC-1), whereas there is no Spanish territory east of 7°30'E ('physical' UTC+1). Spain's time is the direct result of Franco's Presidential Order (published in Boletín Oficial del Estado of 8 March 1940)[14] abandoning Greenwich UTC time in favour of UTC+1 effective 23:00 16 March 1940. This is indeed an excellent example of the aforementioned political criteria used in the drawing of time zones: the time change was passed "in consideration of the convenience from the national time marching in step according to that of other European countries".[15][16] The Presidential Order, most likely enacted to be in synchrony with allies nazi Germany and fascist Italy, included in its 5th article a provision for its future phase out[16] which never took place. Due to this political decision Spain is two hours ahead of its local mean time during the summer (one hour ahead in winter), which probably explains the notoriously late schedule for which the country is known.[17]
  • Belgium
  • Netherlands
  • Most of France, incl. the cities of Paris, Marseilles and Lyon. Only small parts of Alsace, Lorraine and Provence are east of 7°30'E ('physical' UTC+1).

See also

Notes

  1. ^ What is GMT? at the BBC Radio World Service
  2. ^ Legal time in the UK is still "Greenwich mean time" (without capitalisation), according to the Interpretation Act 1978 (with an exception made for those periods when the Summer Time Act 1972 orders a hour's shift for daylight saving), see also the legal section in this article. Mean time at Greenwich was the prototype of Universal Time, as from the late 19th century, see the terms of the International Meridian Conference. Thus UT1, as a form of UT equalized for polar motion, is still a measure of mean time at Greenwich. The widely used time signals based on UTC are kept within 0.9 seconds of UT1, see 'History' section of this article). (The practice of referring to the time signals as 'GMT' is therefore only quantitatively correct to 0.9 second, and is informal and unofficial, see the 'time zone' section of this article.) UTC+0 is explained elsewhere in this article (links from map caption) as equal to Western European Time (WET). Thus WET is within 0.9 second of Greenwich mean time, and (within that tolerance) can fairly be described as 'substantially equivalent'.
  3. ^ a b Myers (2007).
  4. ^ UT1 as explained on IERS page
  5. ^ Howse 1997, p. 178
  6. ^ a b Dumortier, Hannelore, & Loncke (n.d.)
  7. ^ Seago & Seidelmann (c. 2001)
  8. ^ Interpretion Act 1978
  9. ^ Standard Time (Amendment) Act, 1971 (Ireland)
  10. ^ Interpretation Act 2005, part iv sec. 18
  11. ^ Interpretation Act, R.S.C. 1985, c. I-21 -- this refers to 'standard time' for the several provinces, defining each in relation to 'Greenwich time', but does not use the expression 'Greenwich mean time'.
  12. ^ "'It’s a little known but interesting fact that the Greenwich Time Signal no longer gives Greenwich Mean Time,' said the NPL’s John Chambers. 'Since 1972, all the time signals in the world have been based on atomic time.'" Six pip salute. (1999).
  13. ^ Standard Time Act, 1968.
  14. ^ "BOE Orden sobre adelanto de la hora legal en 60 minutos". http://www.boe.es/g/es/bases_datos/tifs.php?coleccion=gazeta&ref=1940/02362&anyo=1940&nbo=68&lim=A&pub=BOE&pco=1675&pfi=1676. Retrieved 2 December 2008. 
  15. ^ "B.O.E. #68 03/08/1940 p.1675". http://www.boe.es/datos/imagenes/BOE/1940/068/A01675.tif. Retrieved 2 December 2008. 
  16. ^ a b "B.O.E. #68 03/08/1940 p.1676". http://www.boe.es/datos/imagenes/BOE/1940/068/A01676.tif. Retrieved 2 December 2008. 
  17. ^ "Hábitos y horarios españoles". http://horariosenespana.com/publicaciones/espana-en-hora/habitos-y-horarios-espanoles.php#siete. Retrieved 27 November 2008. 

References

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  1. Greenwich Mean Time

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